Medical imaging, including X-ray, magnetic resonance (MR), computed tomography (CT), ultrasound, and various combinations of these techniques are utilized to provide images of internal patient structure for diagnostic purposes as well as for interventional procedures. One application of medical imaging (e.g., 3-D imaging) is in the detection and/or treatment of prostate cancer. According to the National Cancer Institute (NCI), a man's chance of developing prostate cancer increases drastically from 1 in 10,000 before age 39 to 1 in 45 between 40 to 59 and 1 in 7 after age 60. The overall probability of developing prostate cancer from birth to death is close to 1 in 6.
Traditionally either elevated Prostate Specific Antigen (PSA) level or Digital Rectal Examination (DRE) has been widely used as the standard for prostate cancer detection. For a physician to diagnose prostate cancer, a biopsy of the prostate must be performed. This is done on patients that have either high PSA levels or an irregular digital rectal exam (DRE), or on patients that have had previous negative biopsies but continue to have elevated PSA. Biopsy of the prostate requires that a number of tissue samples (i.e., cores) be obtained from various regions of the prostate. For instance, the prostate may be divided into six regions (i.e., sextant biopsy), apex, mid and base bilaterally, and one representative sample is randomly obtained from each sextant. Such random sampling continues to be the most commonly practiced method although it has received criticism in recent years on its inability to sample regions where there may be significant volumes of malignant tissues resulting in high false negative detection rates. Further using such random sampling it is estimated that the false negative rate is about 30% on the first biopsy. 3-D Transrectal Ultrasound (TRUS) guided prostate biopsy is a commonly used method to guide biopsy when testing for prostate cancer, mainly due to its ease of use and low cost.
Recently, it has been suggested that TRUS guidance may also be applicable for targeted focal therapy (TFT). In this regard, adoption of TFT for treatment of prostate cancer has been compared with the evolution of breast cancer treatment in women. Rather than perform a radical mastectomy, lumpectomy has become the treatment of choice for the majority of early-stage breast cancer cases. Likewise, some commentators believe that accurate targeting and ablation of cancerous prostate tissue (i.e., TFT) may eventually replace prostatectomy and/or whole gland treatment as the first choice for prostate treatment. Such targeted treatment has the potential to alleviate side effects of current treatment including, incontinence and/or impotence. Such commentators typically agree that the ability to visualize malignant or cancerous tissue during treatment will be of importance to achieve the accuracy of targeting necessary to achieve satisfactory results.
While TRUS provides a convenient platform for real-time guidance for either biopsy or therapy, it is believed that some malignant tissues can be isoechoic in TRUS. That is, differences between malignant cells and surrounding healthy tissue may not be discernable in a standard ultrasound image. Accordingly, using a standard TRUS image a sole means of guidance has not allowed for visually identifying potentially malignant tissue. Further, speckle and shadows make ultrasound images difficult to interpret, and many cancers are often undetected even after saturation biopsies that obtain several (>20) needle samples. To improve the identification of potentially cancerous regions for biopsy or therapy procedures, it has been proposed to combine different pre-acquired imaging modalities (e.g., MRI, CT etc.), which may provide improved tissue contrast, with a live TRUS image during biopsy or therapy.
Imaging modalities like computed tomography (CT) and magnetic resonance imaging (MRI) can provide information that previously could not be derived from standard TRUS imaging alone. While CT lacks good soft tissue contrast to help detect abnormalities within the prostate, it can be helpful in finding extra-capsular extensions when soft tissue extends to the periprostatic fat and adjacent structures, and seminal vesicle invasions. MRI is generally considered to offer the best soft tissue contrast of all imaging modalities. Both anatomical (e.g., T1, T2) and functional MRI, e.g. dynamic contrast-enhanced (DCE), magnetic resonance spectroscopic imaging (MRSI) and diffusion-weighted imaging (DWI) can help visualize and quantify regions of the prostate based on specific attributes. Stated otherwise, such different imaging modalities may allow for locating suspect regions or lesions within the prostate even when such regions/lesions are isoechoic.
Unfortunately, use of pre-acquired images, from different imaging modalities, with a live TRUS image provides a number of logistic problems. Specifically, use of other different imaging modalities such as MRI has required a patient to attend a separate procedure during which images of the other imaging modality are acquired. Once such images are acquired, (e.g., an MRI or CT image) such images must be registered with a live TRUS image acquired during a biopsy or therapy procedure. Registration of images obtained from different modalities creates a number of complications. This is especially true in soft tissue applications where the shape of an object in two images may change between acquisitions of each image. Further, in the case of prostate imaging the frame of reference (FOR) of the acquired images is typically different. That is, multiple MM volumes are obtained in high resolution transverse, coronal or sagittal planes respectively. These planes are usually in rough alignment with the patient's head-toe, anterior-posterior or left-right orientations. In contrast, TRUS images are often acquired while a patient lies on his side in a fetal position by reconstructing multiple rotated samples 2D frames to a 3D volume. The 2D image frames are obtained at various instances of rotation of the TRUS probe after insertion in to the rectal canal. The probe is inserted at an angle (approximately 30-45 degrees) to the patient's head-toe orientation. As a result the gland in MM and TRUS will need to be rigidly aligned because their relative orientations are unknown at scan time. Also the two glands would have to be compensated for the different gland shapes (non-rigid alignment) due to various factors like bladder filling, pressure of the ultrasound probe on the prostate, etc. It is against this background that the present invention has been developed.